Control circuit with fast dynamic response for power converters

ABSTRACT

A control circuit of a power converter is provided. It comprises a voltage detection circuit detecting a reflected signal for generating a voltage-loop signal. A current detection circuit detects a current of a transformer for generating a current-loop signal. An oscillator generates an oscillation signal in accordance with an output load of the power converter. A PWM circuit generates a switching signal according to the voltage-loop signal, the current-loop signal and the oscillation signal for regulating an output of the power converter. A load detection circuit receives a detection signal through an signal-transfer device for increasing a switching frequency of the switching signal. The detection signal is generated once the output is lower than a low-voltage threshold. The oscillation signal determines the switching frequency of the switching signal. The control circuit reduces the voltage drop of the output when the output load is changed.

REFERENCE TO RELATED APPLICATION

This Application is based on Provisional Application Ser. No. 61/680,127, filed 6 Aug. 2012, currently pending.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to a power converter, and more specifically relates to a control circuit of the power converter.

2. Description of the Related Art

The primary side regulation technologies had been disclosed in many prior arts such as, “PWM controller regulating output voltage and output current in primary side”, U.S. Pat. No. 6,721,192; “Primary-side controlled flyback power converter”, U.S. Pat. No. 6,853,563; “Close-loop PWM controller for primary-side controlled power converters”, U.S. Pat. No. 7,016,204; and “Switching control circuit having off-time modulation to improve efficiency of primary-side controlled power supply”, U.S. Pat. No. 7,362,593; etc. The drawback of these prior arts is the slow response to the change of the output load, particularly when the power converter is operated in light load or no load.

FIG. 1 is a circuit schematic of a prior art of a primary side regulation power converter. A transformer 10 has a primary winding N_(P), a secondary winding N_(S), and an auxiliary winding N_(A). A terminal of the primary winding N_(P) is coupled to an input terminal of the power converter to receive an input voltage V_(IN). The secondary winding N_(S) generates an output voltage V_(O) at an output terminal of the primary side regulation power converter via a diode 40 and a capacitor 45. A transistor 20 is coupled to the other terminal of the primary winding N_(P) to switch the transformer 10 for transferring the energy from the input terminal to the output terminal of the power converter. When the transistor 20 is turned on, the transformer 10 is magnetized. The transformer 10 is demagnetized and the energy of the transformer 10 is delivered to the capacitor 45 via the diode 40 for generating the output voltage V_(O) at the output terminal once the transistor 20 is turned off. Meanwhile, a reflected voltage V_(AUX) is generated at the auxiliary winding N_(A) of the transformer 10. The secondary winding N_(S) is proportional to the primary winding N_(P). The output voltage V_(O) is thus correlated to the input voltage V_(IN).

A voltage divider developed by resistors 51 and 52 is coupled to the auxiliary winding N_(A) for generating a reflected signal V_(S) in response to the reflected voltage V_(AUX) of the auxiliary winding N_(A). The reflected signal V_(S) is thus correlated to the reflected voltage V_(AUX). The auxiliary winding N_(A) is proportional to the primary winding N. The reflected voltage V_(AUX) is thus correlated to the output voltage V_(O) during the period that the transformer 10 is demagnetizing. In other words, the reflected signal V_(S) is also correlated to the output voltage V_(O). A control circuit 50 is coupled to sample the reflected signal V_(S) of the voltage divider for generating a switching signal S_(W). The control circuit 50 is further coupled to receive a current-sense signal V_(CS) for adjusting the switching signal S_(W). The switching signal S_(W) controls the transistor 20 to switch the transformer 10 and regulate the output voltage V_(O).

A current-sense device 30, such as a resistor, is coupled between the transistor 20 and the ground. The current-sense device 30 senses a switching current I_(P) of the transformer 10 and generates the current-sense signal V_(CS) in response to the switching current I_(P).

The reflected signal V_(S) is correlated to the output voltage V_(O) during the period that the transformer 10 is demagnetizing. Therefore, the information of the output voltage V_(O) can only be sampled when the transformer 10 is switched on/off. Because the switching frequency of the switching signal S_(W) is decreased during the light load condition or the no load condition for reducing the power loss of the power converter, the information of the output voltage V_(O) can not be detected in between the switching of the transformer 10. Thus, a significant voltage drop of the output voltage V_(O) would be happened when the output load of the power converter is increased rapidly from the light load to the heavy load.

FIG. 2 shows the waveforms of the switching signal S_(W), the output load LOAD and the output voltage V_(O) of the primary side regulation power converter shown in FIG. 1. The level of the output voltage V_(O) is decreased significantly when the output load is increased suddenly during the light load condition. In the light load condition, the switching period of the switching signal S_(W) is long for reducing the switching loss of the power converter. Because the response of the power converter is slow to the change of the output load, the significant voltage drop ΔV_(O1) of the output voltage V_(O) would be happened when the output load of the power converter is increased rapidly from the light load to the heavy load.

BRIEF SUMMARY OF THE INVENTION

The objective of the present invention is to provide a control circuit with fast dynamic response for power converters, it reduces the voltage drop of the output when the output load is changed.

A control circuit of a power converter according to the present invention comprises a voltage detection circuit, a current detection circuit, an oscillator, a PWM circuit, and a load detection circuit. The voltage detection circuit is coupled to detect a reflected signal for generating a voltage-loop signal. The current detection circuit is coupled to detect a current of a transformer of the power converter for generating a current-loop signal. The oscillator generates an oscillation signal in accordance with an output load of the power converter. The PWM circuit generates a switching signal according to the voltage-loop signal, the current-loop signal and the oscillation signal for regulating an output of the power converter. The load detection circuit is coupled to receive a detection signal through a signal-transfer device for increasing a switching frequency of the switching signal. The detection signal is generated once the output of the power converter is lower than a low-voltage threshold. The oscillation signal is coupled to determine the switching frequency of the switching signal. The switching signal is used for switching the transformer through a transistor.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide further understanding of the invention, and are incorporated into and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.

FIG. 1 is a circuit schematic of a prior art of the primary side regulation power converter.

FIG. 2 shows the waveforms of the switching signal S_(W), the output load LOAD and the output voltage V_(O) of the primary side regulation power converter shown in FIG. 1.

FIG. 3 is a circuit schematic of an embodiment of the power converter according to the present invention.

FIG. 4 is a circuit schematic of an embodiment of the control circuit according to the present invention.

FIG. 5 is a circuit schematic of an embodiment of the oscillator in accordance with the present invention.

FIG. 6 is a circuit schematic of an embodiment of the PWM circuit in accordance with the present invention.

FIG. 7 is a circuit schematic of an embodiment of the boost circuit in accordance with the present invention.

FIG. 8 shows waveforms of the switching signal S_(W), the output load LOAD, the detection signal S_(DET), and the output voltage V_(O) in accordance with the present invention.

FIG. 9 is a circuit schematic of another embodiment of the control circuit in accordance with the present invention.

FIG. 10 is a circuit schematic of an embodiment of the oscillator of the control circuit shown in FIG. 9 in accordance with the present invention.

FIG. 11 is a circuit schematic of an embodiment of the pulse generator in accordance with the present invention.

FIG. 12A is a circuit schematic of second embodiment of the power converter according to the present invention.

FIG. 12B is a circuit schematic of third embodiment of the power converter according to the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 3 is a circuit schematic of an embodiment of the power converter according to the present invention. The power converter comprises the transformer 10 having the primary winding N_(P), the secondary winding N_(S), and the auxiliary winding N_(A). The primary winding N_(P) is coupled to receive the input voltage V_(IN) via an input terminal of the power converter. The secondary winding N_(S) generates the output voltage V_(O) at an output terminal of the power converter via the diode 40 and the capacitor 45. The transistor 20 is coupled to switch the transformer 10 for transferring the energy from the input voltage V_(IN) to the output voltage V_(O). When the transistor 20 is turned on, the transformer 10 is magnetized and the transformer's switching current I_(P) is flowed through the transistor 20. The current-sense device 30 coupled between the transistor 20 and the ground senses the switching current I_(P) to generate the current-sense signal V_(CS) coupled to a control circuit 100A/100B. Therefore, the current-sense signal V_(CS) is correlated to the switching current I_(P) of the transformer 10.

The transformer 10 is demagnetized and the energy of the transformer 10 is delivered to the output voltage V_(O) once the transistor 20 is turned off. Meanwhile, the reflected voltage V_(AUX) is generated at the auxiliary winding N_(A) of the transformer 10. The control circuit 100A/100B is coupled to the voltage divider developed by the resistors 51 and 52 to sample the reflected signal V_(S). The reflected signal V_(S) is correlated to the reflected voltage V_(AUX). In other words, the control circuit 100A/100B is coupled to the auxiliary winding N_(A) to sample the reflected voltage V_(AUX) through the voltage divider. The auxiliary winding N_(A) is proportional to the primary winding N_(P). The reflected voltage V_(AUX) is thus correlated to the output voltage V_(O) during the period that the transformer 10 is demagnetizing. That is to say, the reflected signal V_(S) is correlated to the reflected voltage V_(AUX), the output voltage V_(O) and the output load during the period that the transformer 10 is demagnetizing. A first terminal of the resistor 51 is coupled to the auxiliary winding N_(A). The resistor 52 is coupled between a second terminal of the resistor 51 and the ground.

The control circuit 100A/100B is connected to the joint of the resistors 51 and 52 to sample the reflected signal V_(S) for sampling the reflected voltage V_(AUX) of the transformer 10. The switching signal S_(W) is generated by the control circuit 100A/100B to control the transistor 20 for switching the transformer 10 and regulating the output (output voltage V_(O) and/or output current I_(O)) of the power converter. The control circuit 100A/100B can be a primary side regulation circuit including a voltage loop and a current loop. The switching frequency of the switching signal S_(W) is decreased during the power converter is operated in the light load condition or the no load condition. The power converter is operated in the sleep mode or the burst mode when the load condition is the light load condition or the no load condition.

A signal-transfer device, such as an opto-coupler 70, is utilized to send a detection signal S_(DET) to the control circuit 100A/100B when the output voltage V_(O) is lower than a low-voltage threshold. That is, the detection signal S_(DET) is generated once the output voltage V_(O) of the power converter is lower than the low-voltage threshold. The low-voltage threshold is determined by a reference device 60 (with a reference voltage V_(R)) and a forward-diode voltage V_(F) of the opto-coupler 70 for example. The detection signal S_(DET) is applied to wake up the control circuit 100A/100B for increasing the switching frequency of the switching signal S_(W) when the power converter is operated in the light load condition or the no load condition and the voltage level of the output voltage V_(O) is lower than the low-voltage threshold.

A first terminal of a resistor 61 is coupled to the output terminal of the power converter. The reference device 60 is coupled between a second terminal of the resistor 61 and the opto-coupler 70. The opto-coupler 70 is coupled to the output terminal of the power converter and the control circuit 100A/100B. The reference device 60 can be a Zener diode according to one embodiment of the present invention. Accordingly, the resistor 61, the reference device 60, and the opto-coupler 70 are located in the secondary-side of the transformer 10 for sensing the output load and the output voltage V_(O). Thus, the resistor 61, the reference device 60, and the opto-coupler 70 serve as a secondary-side sensing circuit for sensing the output load and the output voltage V_(O).

FIG. 4 is a circuit schematic of an embodiment of the control circuit 100A according to the present invention. It includes a voltage detection circuit (V-Loop) 110 to generate a voltage feedback signal V_(V) in accordance with the reflected signal V_(S). The voltage feedback signal V_(V) is coupled to a first error amplifier 120 for generating a voltage-loop signal V_(CMP). In other words, the voltage detection circuit 110 is utilized to detect the reflected signal V_(S) for generating the voltage-loop signal V_(CMP).

The voltage detection circuit 110 further generates a demagnetized signal S_(DS) in response to the reflected signal V_(S). The demagnetized signal S_(DS) is coupled to a current detection circuit (I-Loop) 210. The current detection circuit 210 generates a current feedback signal V_(I) in accordance with the current-sense signal V_(CS) and the demagnetized signal S_(DS). The current feedback signal V_(I) is coupled to a second error amplifier 220 for generating a current-loop signal I_(CMP). In other words, the current detection circuit 210 is utilized to detect the switching current I_(P) of the transformer 10 (as shown in FIG. 3) for generating the current-loop signal I_(CMP).

The voltage feedback signal V_(V) generated by the voltage detection circuit 110 is coupled to a negative input terminal of the first error amplifier 120. The first error amplifier 120 is a trans-conductance error amplifier with a first reference signal V_(REF1) according to one embodiment of the present invention. The first reference signal V_(REF1) is supplied to a positive input terminal of the first error amplifier 120. An output terminal of the first error amplifier 120 is coupled to generate the voltage-loop signal V_(CMP) by comparing the voltage feedback signal V_(V) with the first reference signal V_(REF1). Therefore, the first error amplifier 120 generates the voltage-loop signal V_(CMP) in accordance with the reflected signal V_(S). In other words, the level of the voltage-loop signal V_(CMP) is related to the voltage level of the output voltage V_(O) and the output load of the power converter.

A first frequency compensation capacitor 125 is coupled to the output terminal of the first error amplifier 120. The first frequency compensation capacitor 125 is used for the frequency compensation of the voltage-loop signal V_(CMP).

The voltage-loop signal V_(CMP) is further coupled to generate the switching signal S_(W) through a first comparator 130, an NAND gate 250 and a PWM circuit (PWM) 400. A positive input terminal of the first comparator 130 is coupled to the first frequency compensation capacitor 125 and the output terminal of the first error amplifier 120 to receive the voltage-loop signal V_(CMP). A negative input terminal of the first comparator 130 receives a ramp signal R_(MP). The ramp signal R_(MP) is generated by the PWM circuit 400. An output terminal of the first comparator 130 is coupled to a first input terminal of the NAND gate 250 for generating a reset signal RST. The reset signal RST is coupled to the PWM circuit 400 to turn off the switching signal S_(W) and modulate the switching frequency of the switching signal S_(W).

The voltage-loop signal V_(CMP) is also coupled to an oscillator (OSC) 300 to modulate the frequency of an oscillation signal PLS generated by the oscillator 300 and the switching frequency of the switching signal S_(W). Thus, the oscillator 300 generates the oscillation signal PLS in accordance with the output load of the power converter. Further, the frequency of the oscillation signal PLS and the switching frequency of the switching signal S_(W) are decreased in response to the decrease of the voltage-loop signal V_(CMP) and the decrease of the output load of the power converter. The oscillation signal PLS is coupled to the PWM circuit 400 to turn on the switching signal S_(W) and determine the switching frequency of the switching signal S_(W).

The current feedback signal V_(I) is coupled to a negative input terminal of the second error amplifier 220. The second error amplifier 220 is a trans-conductance error amplifier with a second reference signal V_(REF2) according to one embodiment of the present invention. The second reference signal V_(REF2) is supplied to a positive input terminal of the second error amplifier 220. An output terminal of the second error amplifier 220 is coupled to generate the current-loop signal I_(CMP) by comparing the current feedback signal V_(I) with the second reference signal V_(REF2). A second frequency compensation capacitor 225 is coupled to the output terminal of the second error amplifier 220. The second frequency compensation capacitor 225 is used for the frequency compensation of the current-loop signal I_(CMP). The level of the current-loop signal I_(CMP) is related to the output current I_(O) (as shown in FIG. 3) of the power converter.

The current-loop signal I_(CMP) is further coupled to generate the switching signal S_(W) through a second comparator 230, the NAND gate 250 and the PWM circuit 400. A positive input terminal of the second comparator 230 is coupled to the second frequency compensation capacitor 225 and the output terminal of the second error amplifier 220 to receive the current-loop signal I_(CMP). A negative input terminal of the second comparator 230 receives the ramp signal R_(MP). An output terminal of the second comparator 230 is coupled to a second input terminal of the NAND gate 250 for generating the reset signal RST. An output terminal of the NAND gate 250 generates the reset signal RST coupled to the PWM circuit 400 to turn off the switching signal S_(W). The current-loop signal I_(CMP) is utilized to regulate the output current I_(O) of the power converter as a constant. The PWM circuit 400 further receives the current-sense signal V_(CS) for modulating the switching signal S_(W).

The detail operation of the voltage detection circuit 110, the current detection circuit 210, the voltage-loop signal V_(CMP), and the current-loop signal I_(CMP) can be found in “Close-loop PWM controller for primary-side controlled power converters”, U.S. Pat. No. 7,016,204. The detail operation of the frequency modulation for the switching signal S_(W) operated in the light load can be found in “Switching control circuit having off-time modulation to improve efficiency of primary-side controlled power supply”, U.S. Pat. No. 7,362,593.

A load detection circuit receives the detection signal S_(DET) from the opto-coupler 70 (as shown in FIG. 3) for detecting the output load condition and increasing the switching frequency of the switching signal S_(W) when the power converter is operated in the light load condition and the voltage level of the output voltage V_(O) is lower than the low-voltage threshold. The load detection circuit comprises a comparator 140. The detection signal S_(DET) is coupled to a positive input terminal of the comparator 140. A negative input terminal of the comparator 140 receives a threshold V_(T). A current source 141 is coupled between a supply voltage V_(CC) and the positive input terminal of the comparator 140. The current source 141 (such as 10 uA) is coupled to the detection signal S_(DET) and applied to pull high the detection signal S_(DET). In normal operation, the detection signal S_(DET) is pulled low by the opto-coupler 70.

Once the output voltage V_(O) is lower than the low-voltage threshold, the detection signal S_(DET) will be higher than the threshold V_(T), and the comparator 140 will generate a signal S_(T) at an output terminal of the comparator 140. The signal S_(T) is coupled to drive a boost circuit 150. The boost circuit 150 will generate a pulse signal in response to the signal S_(T) to charge the first frequency compensation capacitor 125 and boost the level of the voltage-loop signal V_(CMP). Thus, the detection signal S_(DET) is used to increase the level of the voltage-loop signal V_(CMP) when the output voltage V_(O) is lower than the low-voltage threshold. The frequency of the oscillation signal PLS and the switching frequency of the switching signal S_(W) will be increased in response to the increase of the level of the voltage-loop signal V_(CMP). Accordingly, the frequency of the oscillation signal PLS and the switching frequency of the switching signal S_(W) are modulated by the voltage-loop signal V_(CMP).

During the light load condition, the frequency of the oscillation signal PLS and the switching frequency of the switching signal S_(W) are reduced (such as several Hz). Once the detection signal S_(DET) is generated (logic-high level), the frequency of the oscillation signal PLS and the switching frequency of the switching signal S_(W) will be increased immediately (such as >20K Hz) to reduce the voltage drop of the output voltage V_(O). The waveform of the output voltage V_(O) is shown in FIG. 8. Thus, during the power converter is operated in the light load condition, the detection signal S_(DET) is coupled to increase the frequency of the oscillation signal PLS and the switching frequency of the switching signal S_(W).

As shown in FIG. 8, the power converter is operated in the light load condition, and the power converter is operated in the sleep mode or the burst mode for decreasing the switching frequency of the switching signal S_(W). The detection signal S_(DET) is generated (logic-high level) when the output voltage V_(O) is lower than the low-voltage threshold due to the output load is increased suddenly during the power converter is operated in the light load condition. The detection signal S_(DET) is applied to wake up the control circuit 100A/100B (as shown in FIG. 3) for modulating the switching signal S_(W). The switching frequency of the switching signal S_(W) is increased. Therefore, the response of the control circuit 100A/100B is fast that can reduce the voltage drop ΔV_(O2) of the output voltage V_(O). As shown in FIG. 8, the voltage drop ΔV_(O2) is smaller than the voltage drop ΔV_(O1) shown in FIG. 2.

FIG. 5 is a circuit schematic of an embodiment of the oscillator 300 in accordance with the present invention. A voltage-to-current converter (V-I) 305 generates a current I_(OSC) in accordance with the voltage-loop signal V_(CMP). The level of the current I_(OSC) is proportional to the level of the voltage-loop signal V_(CMP). The current I_(OSC) is coupled to charge and discharge a capacitor 325 through current mirrors, a charge switch 321 and a discharge switch 322 for generating the oscillation signal PLS.

A first current mirror comprises transistors 311 and 313. A second current mirror comprises transistors 311 and 312. A third current mirror includes transistors 314 and 315. The first current mirror generates a discharge current I_(D) for discharging the capacitor 325. Gates of the transistors 311 and 313 are coupled together, and sources of the transistors 311 and 313 are coupled to the ground. A drain of the transistor 311 is coupled to the gates of the transistors 311 and 313. The drain of the transistor 311 further receives the current I_(OSC). A drain of the transistor 313 generates the discharge current I_(D). The second current mirror generates a current I₃₁₂. A gate of the transistor 312 is coupled to the gate of the transistor 311. A source of the transistor 312 is coupled to the ground. A drain of the transistor 312 generates the current I₃₁₂.

The third current mirror generates a charge current I_(C) in response to the current I₃₁₂ for charging the capacitor 325. Sources of the transistors 314 and 315 are coupled to the supply voltage V_(CC). Gates of the transistors 314 and 315 are coupled together. A drain of the transistor 314 is coupled to the drain of the transistor 312 to receive the current I₃₁₂. The drain of the transistor 314 is further coupled to the gates of the transistors 314 and 315. A drain of the transistor 315 generates the charge current I_(C).

The charge switch 321 is coupled between the drain of the transistor 315 and a first terminal of the capacitor 325. The charge current I_(C) charges the capacitor 325 when the charge switch 321 is turned on. The discharge switch 322 is coupled between the drain of the transistor 313 and the first terminal of the capacitor 325. The discharge current I_(D) discharges the capacitor 325 when the discharge switch 322 is turned on. A signal V_(SAW) is thus generated at the capacitor 325. A second terminal of the capacitor 325 is coupled to the ground.

The oscillator 300 further comprises comparators 331, 332, NAND gates 341, 342, inverters 345 and 346 for generating the oscillation signal PLS. A positive input terminal of the comparator 331 is coupled to receive a threshold V_(H), and a negative input terminal of the comparator 331 is coupled to receive the signal V_(SAW). The comparator 331 compares the signal V_(SAW) with the threshold V_(H). A negative input terminal of the comparator 332 is coupled to receive a threshold V_(L), and a positive input terminal of the comparator 332 is coupled to receive the signal V_(SAW). The comparator 332 compares the signal V_(SAW) with the threshold V_(L).

A first input terminal of the NAND gate 341 is coupled to an output terminal of the comparator 331. An output terminal of the NAND gate 342 is coupled to a second input terminal of the NAND gate 341. A first input terminal of the NAND gate 342 is coupled to an output terminal of the comparator 332. A second input terminal of the NAND gate 342 is coupled to an output terminal of the NAND gate 341. An input terminal of the inverter 345 is coupled to the output terminal of the NAND gate 341. An output terminal of the inverter 345 is coupled to an input terminal of the inverter 346 and a control terminal of the charge switch 321. The charge switch 321 is thus controlled by an output signal of the inverter 345. An output terminal of the inverter 346 is coupled to a control terminal of the discharge switch 322 and the PWM circuit 400 (as shown in FIG. 4). The oscillation signal PLS is generated at the output terminal of the inverter 346. That is to say, the discharge switch 322 is controlled by the oscillation signal PLS.

According to above, the voltage-to-current converter 305 generates the current I_(OSC) in accordance with the voltage-loop signal V_(CMP) to charge and discharge the capacitor 325 for generating the oscillation signal PLS. When the level of the voltage-loop signal V_(CMP) is decreased, the frequency of the oscillation signal PLS is decreased.

The oscillator 300 further includes a current source 310. The current source 310 is utilized to determine a minimum charge current and a minimum discharge current for the capacitor 325. Thus, the current source 310 is utilized to determine a minimum frequency of the oscillation signal PLS. A first terminal of the current source 310 is coupled to the supply voltage V_(CC). A switch 304 is coupled between a second terminal of the current source 310 and the drain of the transistor 311.

The voltage-loop signal V_(CMP) is coupled to a negative input terminal of a comparator 301. A positive input terminal of the comparator 301 is coupled to receive a threshold V_(T1). The comparator 301 compares the voltage-loop signal V_(CMP) with the threshold V_(T1). An output terminal of the comparator 301 is coupled to an input terminal of a time-delay circuit (DLY) 302. The time-delay circuit 302 delays an output of the comparator 301. An output terminal of the time-delay circuit 302 is coupled to an input terminal of an inverter 303. An output terminal of the inverter 303 is coupled to control the switch 304.

The switch 304 is coupled to enable the current source 310. The switch 304 is turned on and the current source 310 is enabled when the voltage-loop signal V_(CMP) is higher than the threshold V_(T1). Thus, the minimum frequency of the oscillation signal PLS and the switching signal S_(W) can be a first frequency F₁ (such as 1.2 KHz). In other words, the minimum frequency is the first frequency F₁ when the level of the voltage-loop signal V_(CMP) is higher than the threshold V_(T1). The first frequency F₁ is determined by the current I_(OSC) and the current of the current source 310. That is, the first frequency F₁ is determined by a first level of the voltage-loop signal V_(CMP) , and the first level of the voltage-loop signal V_(CMP) is higher than the threshold V_(T1).

When the voltage-loop signal V_(CMP) is lower than the threshold V_(T1), then the comparator 301 will turn off the switch 304 (disable the current source 310) via the time-delay circuit 302 (such as with a 10 msec delay) and the inverter 303. Therefore, the minimum frequency of the oscillation signal PLS is determined only by the current I_(OSC). Thus, the minimum frequency of the oscillation signal PLS and the switching signal S_(W) can be lower than a second frequency F₂ (such as 20 Hz) or can be the second frequency F₂. The second frequency F₂ is lower than the first frequency F₁.

FIG. 6 shows a circuit schematic of an embodiment of the PWM circuit 400 in accordance with the present invention. An input terminal D of a flip-flop 425 is coupled to receive the supply voltage V_(CC). The oscillation signal PLS is coupled to a clock input terminal CK of the flip-flop 425 via an inverter 410 to turn on the switching signal S_(W). An output terminal Q of the flip-flop 425 is coupled to an input terminal of an output buffer 430. An output terminal of the output buffer 430 generates the switching signal S_(W). The oscillation signal PLS determines the switching frequency of the switching signal S_(W).

An inverter 451, a transistor 452, a current source 453, a capacitor 460 and a comparator 470 develop a circuit to limit the maximum on-time of the switching signal S_(W) and generate the ramp signal R_(MP). An output signal generated by the flip-flop 425 is coupled to a gate of the transistor 452 through the inverter 451 to drive the transistor 452. One terminal of the current source 453 is coupled to the supply voltage V_(CC). The other terminal of the current source 453 is coupled to a drain of the transistor 452 and a terminal of the capacitor 460. A source of the transistor 452 and the other terminal of the capacitor 460 are coupled to the ground. The capacitor 460 is charged by the current source 453 when the level of the output signal of the flip-flop 425 is a high level and the transistor 452 is turned off.

A signal generated by the capacitor 460 is coupled to a negative input terminal of the comparator 470. A threshold V_(T2) is supplied to a positive input terminal of the comparator 470. The comparator 470 compares the signal of the capacitor 460 with the threshold V_(T2). An output terminal of the comparator 470 is coupled to a first input terminal of an AND gate 420. The reset signal RST is coupled to a second input terminal of the AND gate 420. An output terminal of the AND gate 420 is coupled to a reset input terminal R of the flip-flop 425. That is to say, an output signal of the comparator 470 and the reset signal RST is coupled to rest the flip-flop 425 for turning off the switching signal S_(W) through the AND gate 420.

The PWM circuit 400 further comprises an adder 480. The signal generated at the capacitor 460 and the current-sense signal V_(CS) is coupled to the adder 480 for generating the ramp signal R_(MP). Thus, the signal generated at the capacitor 460 is associated with the current-sense signal V_(CS) to generate the ramp signal R_(MP) for the pulse width modulation (PWM).

FIG. 7 is a circuit schematic of an embodiment of the boost circuit 150 in accordance with the present invention. The boost circuit 150 comprises a flip-flop 151, a current source 160, a switch 165, a comparator 175, and an inverter 176. The supply voltage V_(CC) is supplied to an input terminal D of the flip-flop 151. The signal S_(T) is coupled to a clock input terminal CK of the flip-flop 151. An output terminal Q of the flip-flop 151 is coupled to control the switch 165. The switch 165 is coupled between the current source 160 and the first frequency compensation capacitor 125 (as shown in FIG. 4). The current source 160 is further coupled to the supply voltage V_(CC). The signal S_(T) is coupled to turn on the switch 165 via the flip-flop 151. The current source 160 is coupled to charge the first frequency compensation capacitor 125 (the voltage-loop signal V_(CMP)) through the switch 165.

The voltage-loop signal V_(CMP) is coupled to a positive input terminal of the comparator 175. A negative input terminal of the comparator 175 is coupled to receive a low threshold V_(RT1). The comparator 175 is utilized to compare the voltage-loop signal V_(CMP) with the low threshold V_(RT1). An output terminal of the comparator 175 is coupled to an input terminal of the inverter 176. An output terminal of the inverter 176 is coupled to a reset input terminal R of the flip-flop 151.

Once the level of the voltage-loop signal V_(CMP) is higher than the low threshold V_(RT1), the flip-flop 151 will be reset (to turn off the switch 165) through the comparator 175 and the inverter 176. The low threshold V_(RT1) determines the level of the voltage-loop signal V_(CMP) when the signal S_(T) is enabled. Therefore, this circuit determines the switching frequency (such as 20 KHz) of the switching signal S_(W) when the detection signal S_(DET) is generated (as shown in FIG. 4). Once the level of the voltage-loop signal V_(CMP) is lower than the low threshold V_(RT1), the detection signal S_(DET) is used to increase the level of the voltage-loop signal V_(CMP).

Furthermore, because the flip-flop 151 is reset when the voltage-loop signal V_(CMP) is higher than the low threshold V_(RT1), the generation of the detection signal S_(DET) will not change the level of the voltage-loop signal V_(CMP) and not increase the switching frequency of the switching signal S_(W). That is to say, the generation of the detection signal S_(DET) can increase the switching frequency of the switching signal S_(W) only when the output load of the power converter is lower than a low-load threshold. The low-load threshold is correlated with the low threshold V_(RT1).

FIG. 9 is a circuit schematic of another embodiment of the control circuit in accordance with the present invention, comparing the control circuit 100B of this embodiment with the control circuit 100A shown in FIG. 4, the control circuit 100B of this embodiment does not have the boost circuit 150, in which the output terminal of the comparator 140 generates the signal S_(T) that is coupled directly to an oscillator 350 (OSC) for increasing the frequency of the oscillation signal PLS and the switching frequency of the switching signal S_(W) when the detection signal S_(DET) is higher than the threshold V_(T).

FIG. 10 shows a circuit schematic of an embodiment of the oscillator 350 of the control circuit 100B shown in FIG. 9 in accordance with the present invention. The difference between the oscillator 350 and the oscillator 300 shown in FIG. 5 is that the oscillator 350 further comprises current sources 317, 351, a switch 352, and a pulse generator 360 and lacks of the comparator 301, the time-delay circuit 302, the inverter 303, the switch 304, the current source 310, the transistors 312, 314, and 315. The different circuits in the oscillator 350 from the oscillator 300 (as shown in FIG. 5) is described as below. The other circuits of the oscillator 350 are the same as the circuits of the oscillator 300 shown in FIG. 5, so here is no detailed description about them.

The oscillation signal PLS and the signal S_(T) are coupled to the pulse generator 360. The pulse generator 360 generates a discharge signal S_(D) in response to the oscillation signal PLS and the signal S_(T). The discharge signal S_(D) is coupled to control the switch 352. The switch 352 is coupled between the current source 351 and the drain of the transistor 311. The current source 351 is further coupled to the supply voltage V_(CC). In other words, the oscillation signal PLS and the signal S_(T) are coupled to enable the switch 352 via the pulse generator 360.

The current source 351 is coupled to discharge the capacitor 325 via the switch 352 and the first current mirror developed by the transistors 311 and the 313. The frequency of the oscillation signal PLS is increased in response to the increase of the discharge current I_(D) for the capacitor 325. The current source 317 is coupled between the supply voltage V_(CC) and the charge switch 321 for charging the capacitor 325.

FIG. 11 shows a circuit schematic of an embodiment of the pulse generator 360 of the oscillator 350 in accordance with the present invention. The pulse generator 360 generates the discharge signal S_(D) in response to the enables of the oscillation signal PLS and the signal S_(T). The oscillation signal PLS and the signal S_(T) are coupled to input terminals of an AND gat 361. An output terminal of the AND gate 361 is coupled to a gate of a transistor 363 to drive the transistor 363. One terminal of a current source 362 is coupled to the supply voltage V_(CC). The other terminal of the current source 362 is coupled to a drain of the transistor 363 and one terminal of a capacitor 365. A source of the transistor 363 and the other terminal of the capacitor 365 are coupled to the ground.

The capacitor 365 is charged by the current source 362 when the transistor 363 is turned off. The capacitor 365 is discharged by the transistor 363 when the oscillation signal PLS and the signal S_(T) are enabled and the transistor 363 is turned on. An input terminal of an inverter 367 is coupled to the capacitor 365. An output terminal of the inverter 367 is coupled to a first input terminal of an AND gate 368. A second input terminal of the AND gate 368 is coupled to the output terminal of the AND gate 361. An output terminal of the AND gate 368 generates the discharge signal S_(D).

FIG. 12A is a circuit schematic of second embodiment of the power converter according to the present invention. The control circuit 100A/100B according to the present invention is also applied to this embodiment. The secondary-side sensing circuit for sensing the output load and the output voltage V_(O) according to this embodiment is different form the secondary-side sensing circuit shown in FIG. 3 for sensing the output load and the output voltage V_(O). The secondary-side sensing circuit of this embodiment is described as below. The other circuits of this embodiment are the same as the circuits of the power converter shown in FIG. 3, so here is no detailed description about them.

The secondary-side sensing circuit of this embodiment comprises a comparator 80, capacitors 81, 82, and a resistor 101. The output voltage V_(O) is coupled to a negative input terminal of the comparator 80. A threshold voltage V_(R2) (the low-voltage threshold) is supplied to a positive input terminal of the comparator 80. The comparator 80 compares the output voltage V_(O) with the threshold voltage V_(R2). Therefore, the comparator 80 is coupled to detect the output voltage V_(O).

The capacitor 82 is coupled between an output terminal of the comparator 80 and the control circuit 100A/100B. A first terminal of the resistor 101 is coupled to the capacitor 82 and the control circuit 100A/100B. A second terminal of the resistor 101 is coupled to the ground. The capacitor 81 is coupled between the current-sense device 30 and a reference potential. The comparator 80 will generate the detection signal S_(DET) through the resistor 101, the capacitors 81 and 82 when the output voltage V_(O) is lower than the threshold voltage V_(R2) (the low-voltage threshold). The capacitor 81 is a Y capacitor of the power converter for reducing the EMI (electrical magnetic interference). The capacitor 82 is operated as the signal-transfer device. The resistor 101 is utilized to pull low the detection signal S_(DET) in normal operation.

FIG. 12B is a circuit schematic of third embodiment of the power converter according to the present invention. The control circuit 100A/100B according to the present invention is also applied to this embodiment. The secondary-side sensing circuit for sensing the output load and the output voltage V_(O) according to this embodiment is different form the secondary-side sensing circuit shown in FIG. 3. The secondary-side sensing circuit of this embodiment is described as below. The other circuits of this embodiment are the same as the circuits of the power converter shown in FIG. 3, so here is no detailed description about them.

The secondary-side sensing circuit of this embodiment comprises the comparator 80, the resistor 101, and a pulse transformer 85, in which the comparator 80 is coupled to detect the output voltage V_(O). The pulse transformer 85 is coupled between the output terminal of the comparator 80 and the control circuit 100A/100B. The first terminal of the resistor 101 is coupled to the pulse transformer 85 and the control circuit 100A/100B. The comparator 80 will generate the detection signal S_(DET) through the resistor 101 and the pulse transformer 85 when the output voltage V_(O) is lower than the threshold voltage V_(R2). The pulse transformer 85 is operated as the signal-transfer device. The resistor 101 is utilized to pull low the detection signal S_(DET) in normal operation.

Although the present invention and the advantages thereof have been described in detail, it should be understood that various changes, substitutions, and alternations can be made therein without departing from the spirit and scope of the invention as defined by the appended claims. That is, the discussion included in this invention is intended to serve as a basic description. It should be understood that the specific discussion may not explicitly describe all embodiments possible; many alternatives are implicit. The generic nature of the invention may not fully explained and may not explicitly show that how each feature or element can actually be representative of a broader function or of a great variety of alternative or equivalent elements. Again, these are implicitly included in this disclosure. Neither the description nor the terminology is intended to limit the scope of the claims. 

What is claimed is:
 1. A control circuit of a power converter, comprising: a voltage detection circuit coupled to detect a reflected signal for generating a voltage-loop signal; a current detection circuit coupled to detect a current of a transformer of the power converter for generating a current-loop signal; an oscillator generating an oscillation signal in accordance with an output load of the power converter; a PWM circuit generating a switching signal according to the voltage-loop signal, the current-loop signal and the oscillation signal for regulating an output of the power converter; and a load detection circuit coupled to receive a detection signal from an opto-coupler for increasing a switching frequency of the switching signal; wherein the detection signal is generated once the output of the power converter is lower than a low-voltage threshold; the oscillation signal is coupled to determine the switching frequency of the switching signal; the switching signal is used for switching the transformer through a transistor.
 2. The circuit as claimed in claim 1, further comprising: an error amplifier coupled to generate the voltage-loop signal in accordance with the reflected signal; wherein the error amplifier has a reference signal and is coupled to a frequency compensation capacitor.
 3. The circuit as claimed in claim 1, further comprising: a boost circuit coupled to the load detection circuit, wherein the load detection circuit drives the boost circuit in response to the detection signal to increase the switching frequency of the switching signal.
 4. The circuit as claimed in claim 3, wherein the boost circuit boosts the level of the voltage-loop signal for increasing the switching frequency of the switching signal, the oscillator generates the oscillation signal in accordance with the voltage-loop signal, and the voltage-loop signal is correlated to the output load.
 5. The circuit as claimed in claim 1, wherein the detection signal is coupled to increase the switching frequency of the switching signal only when the output load is lower than a low-load threshold.
 6. The circuit as claimed in claim 1, wherein the detection signal is coupled to increase the level of the voltage-loop signal when the level of the voltage-loop signal is lower than a low threshold.
 7. The circuit as claimed in claim 1, wherein a frequency of the oscillation signal is decreased in response to the decrease of the output load.
 8. The circuit as claimed in claim 1, wherein the switching frequency of the switching signal is modulated by the voltage-loop signal; the switching signal has a minimum frequency; the minimum frequency is a first frequency determined by a first level of the voltage-loop signal; the minimum frequency is a second frequency when the voltage-loop signal is lower than a threshold after a time delay; the first frequency is higher than the second frequency.
 9. A control circuit of a power converter, comprising: a voltage detection circuit coupled to detect a reflected signal for generating a voltage-loop signal; a current detection circuit coupled to detect a current of a transformer of the power converter for generating a current-loop signal; an oscillator generating an oscillation signal in accordance with an output load of the power converter; a PWM circuit generating a switching signal according to the voltage-loop signal, the current-loop signal and the oscillation signal for regulating an output of the power converter; and a load detection circuit coupled to receive a detection signal from a signal-transfer device for increasing a switching frequency of the switching signal; wherein the detection signal is generated once the output of the power converter is lower than a low-voltage threshold; the oscillation signal is coupled to determine the switching frequency of the switching signal; the switching signal is used for switching the transformer through a transistor.
 10. The circuit as claimed in claim 9, further comprising: an error amplifier coupled to generate the voltage-loop signal in accordance with the reflected signal; wherein the error amplifier has a reference signal and is coupled to a frequency compensation capacitor.
 11. The circuit as claimed in claim 9, wherein the detection signal is coupled to increase the switching frequency of the switching signal only when the output load is lower than a low-load threshold.
 12. The circuit as claimed in claim 9, wherein the detection signal is coupled to increase the level of the voltage-loop signal when the level of the voltage-loop signal is lower than a low threshold.
 13. The circuit as claimed in claim 9, wherein a frequency of the oscillation signal is decreased in response to the decrease of the output load.
 14. The circuit as claimed in claim 9, wherein the switching frequency of the switching signal is modulated by the voltage-loop signal; the switching signal has a minimum frequency; the minimum frequency is a first frequency determined by a first level of the voltage-loop signal; the minimum frequency is a second frequency when the voltage-loop signal is lower than a threshold after a time delay; the first frequency is higher than the second frequency.
 15. The circuit as claimed in claim 9, wherein the signal-transfer device is a capacitor.
 16. The circuit as claimed in claim 9, wherein the signal-transfer device is a pulse transformer.
 17. The circuit as claimed in claim 9, wherein the signal-transfer device is an opto-coupler. 